1. Field of the Invention
The present invention generally relates to screen printing and more particularly to a method for minimizing the distortion of a screening mask used in the screen printing process by controlling the cooling of a screening mask frame holding the screening mask after the screening mask has been cleaned at an elevated temperature and which frame and screen is then to be returned to the screen printing step of the process.
2. Description of Related Art
Screen printing to form a desired pattern on a substrate is very well known in the art. In general, the lower surface of a mask having a desired pattern is placed in contact with a substrate and a fluid material applied to the upper surface of the mask. The fluid material is then forced through the mask pattern (openings) onto the substrate forming the desired pattern. For convenience, the following description will be directed to the manufacture of semiconductor electronic components although it will be appreciated by those skilled in the art that the invention may be used for other screen printing processes.
In the fabrication of multi-layer ceramic substrates (MLC) for making semiconductor devices, conductive metal patterns are screened on individual ceramic greensheets by screen printing. In this process a screening mask, such as a metal mask, is placed in contact with the greensheet and a metallic paste is squeegeed on the greensheet surface through the mask pattern openings. After screening, the greensheets are assembled, aligned and laminated followed by a sintering operation to form a MLC. Fabrication techniques for making semiconductor devices including design, screening equipment and post screening processes are well known in the art.
The ground rules for making semiconductor devices continues to shrink, however, and maintaining tight control of pattern feature size and locations and mask tension of the mask screen becomes increasingly important. One problem in the screening process is that after the screening operation, paste residue is entrapped in the mask features in addition to a surface residue which is left on the screen surface. This requires that the mask be cleaned after one or more screening passes to eliminate/minimize the possibility of defects in subsequently screened patterns. Any defects in the screened paste pattern replicates into the final product causing a defective pattern and rejection of the product.
Using automated screening and mask cleaning processes, mask cleaning is typically preferred after one or more screening passes depending on whether the paste is fast drying or slow drying, otherwise the paste residue on the mask can cause defects in subsequently screened conductive patterns. Screening masks are typically made of metal, such as etched Ni or Mo masks. The screening paste used for defining via and wiring metallurgy patterns on the ceramic greensheets are typically comprised of a metal such as molybdenum, copper, tungsten, nickel, gold, platinum and silver. Some pastes may also contain inorganic fillers such as glass or ceramic powder, dispersed in an organic polymer binder in a high boiling organic solvent vehicle. Other additives include surfactants/dispersants, coloring agents, thickening agents or viscosity modifiers and antioxidants, etc.
In one in-line screening process, the screening mask secured in a frame, usually square, is supported on spaced apart rails whereby the front side and rear side of the frame traverse the space between the rails and the opposed sides of the frame rest on the opposed rails. In this system, the screening mask is secured to the lower side of the frame and the mask is suspended between the spaces of the rails. A greensheet is then positioned adjacent the screening mask and a conductive paste is applied to the upper surface of the screen and squeegeed onto the greensheet surface. After the screening operation the greensheet is removed and the frame and screening mask, which is now coated with excess paste, must be cleaned and the frame and screening mask assembly is moved to the cleaning section.
One technique for cleaning screen masks is by pressure spray cleaning with chemical solutions such as a dilute solution of tetramethyl ammonium hydroxide (TMAH) and/or related quaternary ammonium hydroxides. It is preferred that the mask cleaning be performed using a high pressure spray with a heated TMAH solution since this provides a reduction in cleaning cycle time. In general, and as described in U.S. Pat. No. 6,280,527 to Sachdev et al., which patent is incorporated herein by reference, Mo metal masks carrying conductive paste residue from the screening step are spray cleaned with an aqueous TMAH solution comprising less than 1 weight % TMAH in water. In one process the screening mask is sprayed for about 30 to 45 seconds using a hand-held spray nozzle or with a multi-nozzle spray at between about 50 and 70 psi and between about 130° F. to about 150° F. solution temperature. The mask can also be cleaned in less than about 15 seconds to about 20 seconds using a high pressure spray wash employing a specially designed multi-nozzle system at between about 150 and 200 psi and 130° F. and 180° F. in an automated in-line screening and mask cleaning machine such as described in U.S. Pat. No. 4,483,040, the disclosure of which is incorporated herein by reference. Subsequent to cleaning with TMAH, the screen masks are typically immediately rinsed with water and dried, preferably with forced hot air or nitrogen.
It is at this point in the screen printing process that distortion of the screening mask can occur because of the difference in cooling between the screening mask, which is relatively thin, and the mask frame which holds the screening mask, and which is relatively thick. In general, a screening mask is about 3 to 4 mils and a mask frame is about ⅛ inch thick. During the cooling the screening mask cools faster than the mask frame and the difference in cooling may cause a distortion of the screening mask and screening mask pattern.
Thick-film silk-screen and metal mask printing is used in the microelectronics industry to produce personalized conductive features for microchip packaging and solder paste applications. As ground rules continue to shrink, maintaining tight control of feature location and mask tension becomes increasingly important. Several patents disclose methods for maintaining such control. Japanese Patent No. JP58102768 discloses a method for heating mask frames using resistive heating elements. Japanese Patent No. JP5154986 improves on this patent by enabling both heating and cooling of the frame using fluid heat exchangers embedded within the mask frame. The addition of cooling capability is especially useful because it enables the compensation for rise in ambient temperature or for process-related temperature increases such as mask cleaning operations. Inherent in both of these methods is the requirement for modification of existing mask frames to accommodate the corresponding methods. For the former disclosure, heating elements would need to be affixed to the outside of the mask frame. For the latter, internal coolant passages would need to be formed inside the frame. The latter method would require a frame of sufficient thickness to accommodate said passages. Neither of these methods lends itself to easy implementation on a large inventory of existing mask frames, especially if existing mask frames are relatively thin (less than 0.25 in. thick) mask resulting in oblong vias and other such distortions of the pattern.
Bearing in mind the problems and deficiencies of the prior art, it is therefore an object of the present invention to provide a method of screen printing which minimizes distortion of the screening mask caused by cooling of the screening mask after an elevated temperature cleaning step.
It is another object of the present invention to provide an apparatus for screen printing which minimizes distortion of the screening mask caused by cooling of the screening mask after an elevated temperature cleaning step.
A further object of the present invention is to provide a cooling plate for cooling a mask frame which holds a screening mask.
Still other objects and advantages of the present invention will in part be obvious and will in part be apparent from the specification.